Microsporidia are obligate, intracellular, spore-forming protozoan parasites that belong to the phylum Microspora, which contains approximately 80 genera and more than 700 species. They are ubiquitous in nature and have been recognized in both vertebrates and invertebrates as the cause of a disease called microsporidiosis. The parasites are characterized by the structure of their spores, which have a complex tubular extrusion mechanism used for injecting the infective material (sporoplasm) into the host cells. The parasites are important causes of disease in certain animals, including rabbits, foxes, dogs, and squirrel monkeys.
To date, only four genera of Microsporidia have been implicated as a cause of human disease: Pleistophora, Enterocytozoon, Nosema, and Encephalitozoon. Shadduck, J. A., and Greeley, E.: Microsporidia and human infections. Clin. Micro. Rev. 2:158, 1989. The first case of microsporidiosis in a human was diagnosed in 1959 in a nine year old Japanese boy, on the identification of Encephalitozoon spores in the cerebral spinal fluid and subsequently in the urine. The patient recovered on treatment with sulphisoxazole.
Microsporidial infections are rare in immunocompetent humans. The dramatic recent increase in human immunodeficiency viral (HIV) infections, the causative agent of acquired immunodeficiency syndrome (AIDS), has been accompanied by a dramatic increase in opportunistic infections, including Microsporidial infections, in the host.
Of the four genera of Microsporidia that are known to infect humans, only Nosema spp and Encephalitozoon spp have been noted to cause ocular infection. Cali, A., et al., Corneal microsporidiosis: Characterization and identification. J. Protozool. 38:215S, 1991. Microsporidial keratoconjunctivitis is often recalcitrant to topical medical therapy. Laboratory diagnosis of the disease has been made chiefly through examination of stained smears of conjunctival and corneal scrapings using transmission electron microscopy.
The species Encephalitozoon cuniculi (E. cuniculi) develops within host cells in vacuoles bounded by a membrane that is thought to be of host cell origin. Ocular E. cuniculi infections result in an intractable keratoconjunctivitis that has been reported only in HIV-seropositive individuals who have AIDS.
Yee, et al., reported in 1990 that an HIV-positive patient with bilateral epithelial keratopathy caused by E. cuniculi was not responsive to sulfa drugs, erythromycin, bacitracin, tobramycin, neomycin, polymyxin B, or fluconozole. Complaints by the patient included foreign body sensation, ocular watery discharge, redness of the eye, and later, a burning sensation and photophobia. The infection was ultimately controlled with zidovudine and itraconazole. Yee, R. W., Tio, F. O., Martinez, J. A., Held, K. S., Shadduck, J. A., and Didier, E. S.: Resolution of microsporidial epithelial keratopathy in a patient with AIDS. Ophthalmology 98:196, 1991. However, others have evaluated intraconazole against E. cuniculi and found it not significantly effective against the organism.
Friedberg, et al., reported in 1990 the case histories of three AIDS patients with bilateral coarse superficial epithelial keratitis caused by E. cuniculi. Symptoms included blurred vision, irritation, photophobia, and dryness. Two of the patients did not respond to a number of treatments, and the third obtained comfort and improvement in vision with sulfisoxazole, but no clinical change on ocular examination. Cunjunctival scrapings revealed the presence of numerous protozoa. Friedberg, D. N., Stenson, S. M., Orenstein, J. M., Tierno, P. M., and Charles, N. C.: Microsporidial keratoconjunctivitis in acquired immunodeficiency syndrome. Arch. Ophthalmol. 108:504, 1990.
Metcalf, et al., reported the case history of an AIDS patient with chronic keratoconjunctivitis also caused by E. cuniculi. The patient reported blurred vision and ocular discomfort. Treatment with propamidine isethionate provided temporary resolution of the infection. Metcalfe, T. W., Doran, R. M. L., Rowlands, P. L., Curry, A., and Lacey, C. J. N.: Microsporidial keratoconjunctivitis in a patient with AIDS. Br. J. Ophthalmol. 76:177, 1992. Others, however, have evaluated propamidine against E. cuniculi and found it not significantly effective against the organism.
Until recently, E. cuniculi was thought to be the only species within the genus Encephalitozoon to cause keratoconjunctivitis. In 1991, however, Didier, et al., reported a new Encephalitozoon species, which was designated E. hellem, isolated from three AIDS patients with keratoconjunctivitis. Didier, et al., J. Infect. Dis. 163:617, 1991.
Human ocular infection caused by members of the genus Nosema occurs in Human Immunodeficiency Virus (HIV)-seronegative as well as HIV-seropositive patients and typically results in corneal stromal ulceration both with and without antecedent trauma. Davis, et al., reported the case study of a healthy HIV-seronegative 45-year old man who developed stromal keratitis and iritis in his left eye caused by a protozoa of the Nosema genus. The patient's clinical course was characterized by progressive central discoform keratitis, and recurrent peripheral anterior stromal patchy infiltration with overlying punctate epitheliopathy, and anterior iritis. The patient was ultimately treated by keratoplasty with cryotherapy applied to the recipient edge for 360 degrees at -60 degrees centigrade before corneal trephination in an attempt to eliminate organisms in the corneal periphery.
Acanthamoeba Keratitis, which is caused by the free-living amoeba acanthamoeba, has also become recognized as a very challenging ocular infection in light of its protracted painful clinical course and frequent treatment failures. Symptoms of the infection include marked pain and photophobia with paracentral ring-shaped stromal infiltration occurring in advanced stages of the disease. Acanthamoeba keratitis was first reported in 1974 and remained a rare infection until it became associated with contact lens wear. Risk factors associated with Acanthamoeba keratitis include contaminated tap water, home-prepared saline and chemical disinfection solutions, and minor corneal injury.
In vitro tests have identified a number of compounds that may be useful in the treatment of Acanthamoeba keratitis, including propamidine isothionate, the aminoglycosides neomycin and paromomycin, and imidazole derivatives miconazole, clotrimazole, ketoconazole, and itraconazole. It has also been reported that polyhexamethylene biguanide is effective against Acanthamoeba. Larkin, et al., Ophthalmology 1992; 99:185-191.
In light of the increasing diagnosis of ocular Microsporidia and Acanthamoeba infections, as well as the uncomfortable and sometimes disabling effect of the diseases, it is important to have an effective treatment that is not unduly toxic to the host.
Fumagillin is an antibiotic produced by Aspergillus fumagatus. It has been tested for use as an antiamoebic in humans (but never marketed as such), and is currently used as an antiprotozoal in the control of Nosema apis in honey bees. Hartwig, A. and Przelecka, A.: Nucleic acids in intestine of Apis mellifica infected with Nosema apis and treated with fumagillin DCH: cytochemical and autoradiographic studies. J. Invertebr. Pathol. 18:331, 1971. Jaronski, S. T.: Cytochemical evidence for RNA synthesis inhibition by fumagillin. J. Antibiot. 25:327, 1972.
Shadduck reported in 1980 that fumagillin inhibits the multiplication of E. cuniculi in rabbit and canine cells in cell culture. Fumagillin was found to have no effect on spores or proliferative forms of the parasite in vitro. Shadduck, J. A.: Effect of fumagillin on in vitro multiplication of Encephalitozoon cuniculi. J. protozool. 27:202, 1980. The antibiotic did not reduce the number of organisms in previously infected cell cultures to zero, but prevented infection of host cells when added to the medium simultaneously with the organism. Shadduck concluded that his data suggests that fumagillin would not eliminate all viable E. cuniculi organisms in infected hosts, but might reduce multiplication of the parasite sufficiently to permit other host defense mechanisms to destroy remaining organisms. In a subsequent paper, Shadduck concluded that while fumagillin is effective in preventing the multiplication of E. cuniculi in vitro, the only available formulation is intended for use in honey bees and is toxic to humans. Further, Shadduck, in a review article subsequent to his work with fumagillin, concluded that there is no known treatment for microsporidial infections. Review of Infectious Diseases, Vol 11(2), 203-207, 1989.
It is an object of the present invention to provide an effective treatment for Microsporidial keratoconjunctivitis.
It is another object of the present invention to provide an effective treatment for Acanthomoeba keratoconjunctivitis.
It is a further object of the present invention to provide a topical composition that is effective against Microsporidial and Acanthamoeba keratoconjunctivitis.